Phase cut dimmers are widely used in a range of applications, including but not limited to lighting applications. A conventional phase-cut dimmer uses a switch to switch off or “cut” part of the mains cycle—or more accurately, half-cycle. Either the leading or trailing edge, or even both edges of the phase can be cut, depending on the type of switch used. Normally either a transistor or a triac is used.
Although this is effective for conventional incandescent lighting, phase cut dimmers can be problematic for more energy efficient types of lighting, such as compact fluorescent lights (CFL), tube lighting (TL), or solid-state lighting (SSL), which are not well suited to cope with the “missing” parts of the mains cycle. Such energy efficient types of lighting are often controlled by means of a switched mode power supply, which switches at frequencies much higher than that of either a 50 Hz or 60 Hz mains cycle—typically several kHz up to several MHz.
For phase cut dimmers to be compatible with such types of lighting, the phase cut power needs to be processed to be usable by the lighting circuit. However, it is important that in such processing, that the information regarding the desired level of dimming, which is included in the phase cut signal, is not lost but is recovered as a dimming control level signal.
Conventionally, a control voltage to indicate the desired level of the dimming is generated by making an integrated signal from the dimmer output signal. This signal will have a level which is proportional to the area under a plot of the dimmer output voltage against time, and is thus a function of not just the dimmer phase angle but also of the mains voltage level. So, fluctuations in mains voltage affect the control level, and thus the apparent dimming setting. Moreover, the integrator needs a time constant the order of seconds for the signal to have minimum ripple, which is not compatible with prompt responsiveness to a user when changing the dimmer level.
An alternative solution has been proposed, for instance in International Patent Application Publication number WO2008/112,735 in which a duty cycle signal is generated from the output of the dimmer. The duty cycle signal, which may take the form of a pulse modulated signal, is indicative of the proportion of the mains cycle during which the switch is on. However, this is not representative of the fraction of the mains power which would be supplied during this proportion of the cycle, due to the sinusoidal nature of both the voltage and current supplied.
Moreover, the response of the human eye to a dimmed light level is generally approximately logarithmic rather than linear, so it be would be preferable if the power supply it to the lighting and could be adjusted so as to be nonlinearly, and ideally logarithmically or nearly logarithmically, related to the phase angle of the dimmer.
It is proposed in WO2008/112,735, to provide such a nonlinear response by post-processing the dimming control level signal using a lookup table. However, such a solution is difficult to implement in a primarily analog circuit, and requires both complex circuitry and significant processing power to implement.
It would therefore be desirable to have a circuit in which the nonlinear response is directly derived from the duty cycle signal.